New lint: redundant_sort_by_key.

[pmnoxx]

What it does

I saw an interesting bug today. Apparently the following code is valid.

[3,1,2].sort_by_key(|x|{x;}|);

The extra semicolon makes this function return (), which is valid, but this makes the sorting pointless.

Lint Name

useless_sort_by_key

Category

correctness

Advantage

• This is likely a bug, and should be fixed. I had it happen to my code, once.
• Or it can be just called redundant sort_by_key.

Drawbacks

• I don't see any

Example

[3,1,2].sort_by_key(|x|{x;}|);

Could be written as:

That codes does nothing can be removed.
Or can be fixed, but it may be impossible to figure out what the user intended

[3,1,2].sort_by_key(|x|{x}|);

I think this should be marked as an potential error.

[5225225]

IMO this should apply to most/all _by_key methods.

So you'd be looking for all calls of a specific method with a signature like

fn max_by_key<B, F>(self, f: F) -> Option<Self::Item> where
    B: Ord,
    F: FnMut(&Self::Item) -> B, 

where B is ().

Though I suppose there may be legitimate use cases for a unit _by_key for some methods, so it might be a good idea to restrict it to just some types / be a bit smarter and let people explicitly return unit.